Projectors are used in many scenarios to create a visual display of information on a surface. However, when the projector is not parallel to the surface (e.g., when the line of projection from the projector to the surface is not perpendicular to the surface), the projection may be distorted. For example, when a projector is sitting on a table facing upward toward the projection surface, the projected image appears larger at the top than at the bottom. This distortion effect is commonly referred to as the keystone effect, given that the effect tends to distort the projected image into a trapezoid shape (i.e., the shape of an architectural keystone).